METHOD AND DEVICE FOR IDENTIFYING INTERFERENCE PIGMENTS IN A COATING

Abstract

Described herein is a method for identifying an interference pigment in a coating. Also described herein is a corresponding device and a computer-readable medium.

Claims

1. A method for identifying an interference pigment (103) in a coating (101), comprising at least the following steps: illuminating the coating (101) applied to a surface with light at a first illumination angle (θ.sub.inc1) relative to the normal to the surface, recording a first image of the coating (101) applied to the surface with light at a first aspecular measuring angle (θ.sub.as1), measured in relation to the Bragg angle (θ.sub.spec), assigned to the first illumination angle (θ.sub.inc1);, illuminating the coating (101) applied to the surface at at least one second illumination angle (θ.sub.inc2) different from the first, relative to the normal to the surface, recording at least one second image of the coating (101) applied to the surface at the first aspecular measurement angle, measured in relation to the Bragg angle (θ.sub.spec2), assigned to the at least one second illumination angle (θ.sub.inc2), or a corrected first aspecular measurement angle (θ.sub.as2), measured in relation to the Bragg angle (θ.sub.spec2), assigned to the at least one second illumination angle (θ.sub.inc2) wherein the corrected first aspecular measurement angle (θ.sub.as2) is obtained from the first aspecular measurement angle (θ.sub.as1) by correction in accordance with an optical refraction of the light that has changed at the at least one second illumination angle (θ.sub.inc2)as compared with the first illumination angle (θ.sub.inc1), spatially analyzing and evaluating the first image and the at least one second image, spectrally analyzing and evaluating the first image and the at least one second image, and identifying the interference pigment (103) on the basis of the spatial and spectral analysis and evaluation of the first image and the at least one second image, the first and the at least one second illumination angle (θ.sub.inc1, θ.sub.inc2) and the first aspecular measurement angle (θ.sub.as1) or the corrected first aspecular measurement angle (θ.sub.as2).

2. The method as claimed in claim 1, in which the step of spectrally analyzing and evaluating comprises deriving a respective remission spectrum for the first and the at least one second image.

3. The method as claimed in claim 2, in which the deriving of the respective remission spectrum for the first and the at least one second image is carried out by means of a simulation algorithm.

4. The method as claimed in claim 1, in which at least one camera is used for recording the first and the at least one second image.

5. The method as claimed in claim 1, in which a plurality of second images at a corresponding plurality of second illumination angles of the coating applied to the surface are recorded at the first aspecular measurement angle or the corrected first aspecular measurement angle, in each case measured in relation to the Bragg angle assigned to the respective second illumination angle.

6. The method as claimed in claim 1, which for identifying a number of pigments, is carried out correspondingly a number of times, while the aspecular measurement angle is likewise correspondingly varied a number of times, whereas, for identifying one interference pigment in each case, the respective aspecular measurement angle is kept constant or is just corrected in accordance with the optical refraction of the light that has changed at the at least one second illumination angle as compared with the first illumination angle.

7. A computer-implemented method for identifying an interference pigment (103) in a coating (101), comprising at least the following steps: receiving a first image, wherein the first image of a coating (101) applied to a surface was recorded under illumination of the coating (101) applied to the surface with light at a first illumination angle (θ.sub.inc1) relative to the normal to the surface by an image capturing unit at a first aspecular measurement angle (θ.sub.as), measured in relation to the Bragg angle (θ.sub.spec1) assigned to the first illumination angle, receiving at least one second image, wherein the at least one second image of the coating (101) applied to the surface was recorded under illumination of the coating (101) applied to the surface with light at a second illumination angle (θ.sub.inc2) different from the first, relative to the normal to the surface by an image capturing unit at the first aspecular measurement angle, measured in relation to the Bragg angle (θ.sub.spec2), assigned to the at least one second illumination angle (θ.sub.inc2), or at a corrected first aspecular measurement angle (θ.sub.as2), measured in relation to the Bragg angle, assigned to the at least one second illumination angle, wherein the corrected first aspecular measurement angle (θ.sub.as2) is obtained from the first aspecular measurement angle (θ.sub.as1) by correction in accordance with an optical refraction of the light that has changed at the at least one second illumination angle (θ.sub.as2) as compared with the first illumination angle (θ.sub.as1), carrying out an image analysis for the first and the at least one second image, by using at least one processor, detecting the interference pigment (103) within the first and the at least one second image by a spatially resolved analysis of the images, evaluating, calculating and/or simulating a respective remission spectrum for the first and the at least one second image, and identifying the interference pigment (103) on the basis of the spatial and spectral analysis and evaluation of the first image and the at least one second image, the first and the at least one second illumination angle (θ.sub.inc1, θ.sub.inc2) and the first aspecular measurement angle (θ.sub.as1) or the corrected first aspecular measurement angle (θ.sub.as2).

8. A device, at least comprising: a database, and a processor, which is in communicational connection with the database and is configured for exchanging data with the database, wherein the processor is programmed to perform the following steps: receiving a first image, wherein the first image of a coating (101) applied to a surface was recorded under illumination of the coating (101) applied to the surface with light at a first illumination angle (θ.sub.inc1) relative to the normal to the surface by an image capturing unit at a first aspecular measurement angle (θ.sub.as), measured in relation to the Bragg angle (θ.sub.spec1) assigned to the first illumination angle (θ.sub.inc1), receiving at least one second image, wherein the at least one second image of the coating (101) applied to the surface was recorded under illumination of the coating (101) applied to the surface with light at a second illumination angle (θ.sub.inc2), different from the first, relative to the normal to the surface by an image capturing unit at the first aspecular measurement angle, measured in relation to the Bragg angle (θ.sub.spec2), assigned to the at least one second illumination angle (θ.sub.inc2), or at a corrected first aspecular measurement angle (θ.sub.as2), measured in relation to the Bragg angle (θ.sub.spec2), assigned to the at least one second illumination angle (θ.sub.inc2), wherein the corrected first aspecular measurement angle (θ.sub.as2) is obtained from the first aspecular measurement angle (θ.sub.as1) by correction in accordance with an optical refraction of the light that has changed at the at least one second illumination angle (θ.sub.inc2) as compared with the first illumination angle (θ.sub.inc1), carrying out an image analysis for the first and the at least one second image, detecting the interference pigment (103) within the first and the at least one second image by a spatially resolved analysis of the images, evaluating, calculating and/or simulating a respective remission spectrum for the first and the at least one second image, and identifying the interference pigment (103) on the basis of the spatial and spectral analysis and evaluation of the first image and the at least one second image, the first and the at least one second illumination angle (θ.sub.inc1, θ.sub.inc2) and the first aspecular measurement angle (θ.sub.as1) or the corrected first aspecular measurement angle (θ.sub.as2).

9. The device as claimed in claim 8, which additionally comprises the image capturing unit, which is in communicational connection with the processor and is configured for recording the first and the at least one second image.

10. The device as claimed in claim 8, which is configured for performing a method as claimed in claim 1.

11. A nonvolatile, computer-readable medium, which comprises a computer program with program coding means which are designed to perform the following steps when the computer program runs on the processor of the device as claimed in claim 8: receiving a first image, wherein the first image of a coating (101) applied to a surface was recorded under illumination of the coating (101) applied to the surface with light at a first illumination angle (θ.sub.inc1) relative to the normal to the surface by an image capturing unit at a first aspecular measurement angle (θ.sub.as), measured in relation to the Bragg angle (θ.sub.spec1) assigned to the first illumination angle (θ.sub.inc1), receiving at least one second image, wherein the at least one second image of the coating (101) applied to the surface was recorded under illumination of the coating (101) applied to the surface with light at a second illumination angle (θ.sub.inc2), different from the first, relative to the normal to the surface by an image capturing unit at the first aspecular measurement angle, measured in relation to the Bragg angle (θ.sub.spec2), assigned to the at least one second illumination angle (θ.sub.inc2), or at a corrected first aspecular measurement angle (θ.sub.as2), measured in relation to the Bragg angle (θ.sub.spec2), assigned to the at least one second illumination angle (θ.sub.inc2), wherein the corrected first aspecular measurement angle (θ.sub.as2) is obtained from the first aspecular measurement angle (θ.sub.as1) by correction in accordance with an optical refraction of the light that has changed at the at least one second illumination angle (θ.sub.inc2) as compared with the first illumination angle (θ.sub.inc1), carrying out an image analysis for the first and the at least one second image, detecting the interference pigment (103) within the first and the at least one second image by a spatially resolved analysis of the images, evaluating, calculating and/or simulating a respective remission spectrum for the first and the at least one second image, and identifying the interference pigment (103) on the basis of the spatial and spectral analysis and evaluation of the first image and the at least one second image, the first and the at least one second illumination angle (θ.sub.inc1, θ.sub.inc2) and the first aspecular measurement angle (θ.sub.as1) or the corrected first aspecular measurement angle (θ.sub.as2).

12. A nonvolatile, computer-readable medium, which comprises a computer program with program coding means which are designed to perform a method as claimed in claim 1 when the computer program runs on a computing unit.

13. The device as claimed in claim 8, which is configured for performing a method as claimed in claim 7.

14. A nonvolatile, computer-readable medium, which comprises a computer program with program coding means which are designed to perform a method as claimed in claim 7 when the computer program runs on a computing unit.

15. The method as claimed in claim 4, in which the at least one camera is a multi-angle color camera and/or a hyperspectral camera and/or a camera system equivalent thereto.

16. The method as claimed in claim 4, in which the at least one camera is a system also operating in the non-visible spectral range.

Description

[0060] The invention is schematically represented by exemplary embodiments in the drawing and is further described with reference to the drawing.

[0061] FIG. 1 shows in a schematic representation in FIG. 1a a possible measuring geometry for capturing an image of a coating applied to a surface of a sample platelet with pigment platelets comprised therein, in FIG. 1b possible measuring geometries for capturing a first and at least one second image of a coating applied to a surface of a sample platelet with pigment platelets comprised therein and in a diagram in FIG. 1c a dependence of an aspecular observation angle on an alignment of an interference pigment in relation to the surface of the coating;

[0062] FIG. 2 shows an image of a coating recorded by a camera, recorded at an aspecular measurement angle measured in relation to the Bragg angle assigned to the illumination angle chosen here;

[0063] FIG. 3 shows simulated and normalized remission spectra of a TiO.sub.2 coating in the case of measuring geometries with various illumination angles and an aspecular measurement angle that is (almost) fixed in relation to the respective illumination angles.

[0064] The method according to the invention, the device according to the invention and the computer program product according to the invention can be used both for auto repair paints for cars or car bodies or body styling parts and for other types of coatings, including colorants and industrial finishes. The embodiments of the invention described below are not intended to be restrictive.

[0065] Embodiments of the method according to the invention, the device according to the invention and the computer-readable medium according to the invention can be used in many areas, such as for example for comparing and/or coordinating design and/or fashion products.

[0066] Embodiments of the method according to the invention can be at least partly performed by or implemented in a computer system, which may be an independent unit or comprise one or more external terminals or equipment that communicate with a central computer via a network, such as for example the Internet or an intranet.

[0067] The computer or processor described in the present disclosure and components coupled therewith or integrated therein may therefore be part of a local computer system or a remote computer or an on-line system or combinations thereof.

[0068] The database described in the context of the present disclosure and the computer program described here may be stored or retrievably stored in an internal computer memory or in a nonvolatile computer-readable medium.

[0069] Embodiments of the method according to the invention and/or of the device according to the invention use an image capturing unit, which may be for example a multi-angle color camera or a multi-angle multispectral camera, possibly in combination with a spectrophotometer, whereby improved and simplified results for pigment characterization and sample properties can be produced.

[0070] The following statement applies to the description of the figures that follows: If individual reference signs are not entered in a figure, reference in this respect is made to the other figures and the associated parts of the description.

[0071] The “specular direction” should be understood as meaning the nominal observing direction reflected at the surface of the planar object to be measured, taken from an illuminating direction. A multi-angle color-measuring instrument or generally a multi-angle measuring instrument for measuring a specular variation of a coating in dependence on the illuminating and/or observing (measuring) direction has a number of illuminating directions and a number of observing directions. The measuring plane should be understood as meaning a plane passing through the normal to the instrument and all of the illuminating directions and the observing directions and also the specular direction. All of the indications of angles relate to directions lying within the measuring plane.

[0072] FIG. 1a shows in a schematic representation a beam path 102 for an optical beam which enters a coating 101 at an angle θ.sub.inc and is reflected back in the direction of the surface of the object to be measured by an interference pigment 103 that is present in the coating and is aligned non-parallel to the coating surface. The object to be measured corresponds in this case to a sample platelet with a coating applied to a sample platelet surface.

[0073] According to the invention, it is envisaged to illuminate a surface of the sample platelet that is coated with a coating 101 or a paint of an unknown formulation with light, in particular white light at a certain illumination angle relative to the surface normal, for example at an angle θ.sub.inc of 45° relative to the surface normal. A known conventional clear coat may be applied over the coating 101 of an unknown formulation. Then, with the aid of a sensor or a camera, a spectral radiation that is reflected from the surface of the object to be measured is captured in a spatially resolved manner, in particular in the form of an image, at a certain aspecular measuring angle θ.sub.as, such as for example as15°, i.e. 15° in relation to the Bragg angle θ.sub.spec, where “as” stands for “aspecular”.

[0074] If the coating of an unknown formulation comprises interference pigments 103, those interference pigments 103, i.e. their interference pigment platelets, that are aligned in relation to the surface of the object to be measured in such a way that they reflect specularly in the direction of the aspecular measurement angle θ.sub.as are visible in the image captured. With knowledge of the refractive indices of the surroundings (typically air, n.sub.surroundings=1) and the coating surrounding the interference pigment (typically n.sub.coating=1.5), it is possible to assign injectively to paired combinations of the illuminating direction θ.sub.inc (illuminating angle) and aspecular measuring angle θ.sub.as an orientation α in which an interference pigment flake (platelet) must be oriented in the coating 101 or the paint in order to reflect the illumination or the incident light 102 specularly (θ.sub.pig) in the observing direction θ.sub.spec+θ.sub.as. For the aspecular angle θ.sub.as, in which the interference pigment 103 oriented at a in relation to the surface is specularly (θ.sub.pig) reflected, the following applies according to FIG. 1a in dependence on the angle of incidence of the light on the surface of the object to be measured θ.sub.inc (measured in relation to the normal):

θ.sub.as=arcsin[n.sub.coating/n.sub.surroundings*sin(arcsin(n.sub.surroundings/n.sub.coating*sin(θ.sub.inc))+α)]−θ.sub.inc  (1)

[0075] FIG. 1b shows two possible measuring geometries for capturing a first and a second image of the coating 101 applied to a surface of an object to be measured with pigment platelets 103 comprised therein, only one pigment platelet being shown here for the sake of simplicity. A first measuring geometry comprises an illumination at a position 111, which illuminates the coating 101 at a first angle of incidence θ.sub.inc1, and a camera at a position 121, which records a first image of the coating 101 at a first aspecular angle θ.sub.as1, measured in relation to the Bragg angle θ.sub.spec1, assigned to the first angle of incidence θ.sub.inc1 or in a first observing direction θ.sub.as1+θ.sub.spec1. A second measuring geometry comprises an illumination at a position 112, which illuminates the coating with light at a second angle of incidence θ.sub.inc2, and a camera at a position 122, which records a second image in a second observing direction θ.sub.as1+θ.sub.spec2, wherein, with a changing angle of incidence θ.sub.inc2, the Bragg angle θ.sub.spec2 also correspondingly changes but the aspecular angle θ.sub.as2≈θ.sub.as1 remains almost the same.

[0076] For an angle of incidence θ.sub.inc2 of the light that is changed with respect to a first angle of incidence θ.sub.inc1, it is accordingly the case, on the basis of the above equation (1) for the specular reflection of the interference pigment 103 oriented at α in relation to the surface with respect to a first observing direction θ.sub.as1+θ.sub.spec1 assigned to the first angle of incidence θ.sub.inc1, that also the observing direction θ.sub.as2+θ.sub.spec2 changes. However, in first approximation (small angles θ and α), it is so that θ.sub.as1≈θ.sub.as2, and consequently that the interference pigment 103 continues to be aspecularly reflected at the same aspecular measurement angle (cf. FIG. 1c). In addition, the optics of the cameras used have a certain acceptance angle (numerical aperture), so that in practice minor deviations in θ.sub.as can be tolerated.

[0077] FIG. 1c shows the dependence of the aspecular observation angle θ.sub.as for specular reflection of an interference pigment located in a coating on the alignment α of the interference pigment in relation to the coating surface or the surface of the object to be measured for various illumination angles θ.sub.inc. The alignment α of the interference pigment is indicated in angular degrees [°] on an x axis. The Bragg angle of the interference pigment, i.e. the aspecular angle θ.sub.as, is indicated in angular degrees [°] on a y axis. Curve 151 shows the dependence at an illumination angle θ.sub.inc of 5°, curve 152 shows the dependence at an illumination angle θ.sub.inc of 15°, curve 153 shows the dependence at an illumination angle θ.sub.inc of 25°, curve 154 shows the dependence at an illumination angle θ.sub.inc of 35°, curve 155 shows the dependence at an illumination angle θ.sub.inc of 45°, curve 156 shows the dependence at an illumination angle θ.sub.inc of 55°, curve 157 shows the dependence at an illumination angle of θ.sub.inc of 65°.

[0078] FIG. 2 shows an image recorded by a camera as part of an embodiment of the device according to the invention. In this case, a surface of an object to be measured which comprises a sample platelet on the surface of which a coating comprising interference pigments has been applied was illuminated with white collimated light at a predetermined angle of incidence. The camera with which the image was recorded was in this case arranged at a prescribed aspecular angle in relation to the Bragg angle, given by the angle of incidence, with reference to the normal to the surface. The sparkle points visible here give an indication of interference pigments comprised by the coating, which generally take the form of interference pigment flakes or platelets finely distributed in the coating. In a next step, another, further image, i.e. a second image, is recorded, wherein in this case the surface of the sample platelet coated with the coating is illuminated at another, second angle of incidence and the recording of the image is however recorded at the same first aspecular angle, or the first aspecular measuring angle corrected for optical refraction at boundary surfaces, in relation to the Bragg angle given by the second angle of incidence. Since the aspecular angle is (almost) retained when recording the second image, the same interference pigments within the coating display specular reflection in the direction of the camera, and are consequently visible in both images. On the basis of the (at least almost) retained aspecular angle in relation to the Bragg angles resulting from the respective angles of incidence, the orientation of the interference pigment flakes visible in the respective images, i.e. in the first and the at least one second image, can be derived. After analysis of the respective images, it is therefore possible on the basis of the characteristic variation, which can be derived from the images, of the spectral reflection in dependence on the angle of incidence to clearly infer the interference pigment type with the aid of the database.

[0079] FIG. 3 shows simulated and normalized reflection spectra of an individual TiO.sub.2-coated mica platelet, embedded in a standard matrix (constant refractive index n=1.5), for measuring geometries with various angles of incidence. The wavelength of the reflected light is in this case plotted in nanometers on an x axis 201. The normalized reflection is plotted on a y axis 202. The reflection spectrum 211 shows the light reflected from the coated mica platelets (=object to be measured) at an angle of incidence of 25° and a measurement angle of 15° aspecularly in relation to the Bragg angle resulting from the angle of incidence. In this case, a strong reflection can be seen in the region of 460 nm and a smaller local maximum in the region of 575 nm. The reflection spectrum 212 shows the light reflected from the coated mica platelet or its surface at an angle of incidence of 45° and the retained aspecular angle of 15°, measured in relation to the Bragg angle resulting from the angle of incidence. Here, the reflection maximum lies in the region of 450 nm and a smaller local maximum in the region of 560 nm. The reflection spectrum 213 was recorded for an angle of incidence of 65°, wherein the measuring instrument provided for recording the reflection spectrum was arranged at an aspecular angle of 15°. Here, a reflection maximum is obtained in the region of 440 nm and a smaller local maximum in the region of 540 nm. When evaluating the resultant reflection spectra, in particular when considering the variation/change of the spectral reflection or of the spectral reflection profile, which can be derived from the reflection spectra, in dependence on the angle of incidence, which is in each case characteristic of a respective interference pigment type, an interference pigment displaying precisely this reflection profile or precisely this variation can be clearly inferred.